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Cognitive changes and dementia in Parkinson's disease (PD) were largely ignored in early descriptions of the condition. These aspects, however, have become increasingly more recognized and highly relevant, as the life expectancy of patients has increased with modern treatments for motor problems, and better healthcare in general. Consequently, disorders of cognition, which tend to emerge in advanced age, have become more apparent. Over the past decade in particular, there has been greater attention to mental aspects of PD, with increasing research devoted to the epidemiology, clinical characteristics, underlying pathophysiology, and treatment of dementia in PD (PD-D). This chapter summarizes the features and management of dementia associated with PD.

Cognitive impairment can be detected at the time of diagnosis in PD if appropriate neuropsychological tests are administered, even though patients may not have subjective complaints. The presence of dementia at the onset of disease would preclude the diagnosis of PD by current definitions and would qualify the patient as having dementia with Lewy Bodies (DLB). In a community-based study in the United Kingdom, 36% of newly diagnosed patients were found to have cognitive impairment, 1 while 57% of this cohort developed cognitive deficits (described as frontostriatal or posterior cortical in type) within 3.5 (±0.7) years from disease onset. 2 In the Netherlands, 24% of newly presenting cases were reported to have varying degrees of cognitive impairment.3

Both prevalence and incidence of dementia are substantially increased in PD compared with age-matched controls. A meta-analysis of 27 earlier studies found a prevalence rate of 40%.4 A more recent systematic review of 12 selected studies reported a point prevalence of 24–31%, it estimated that 3–4% of all dementias in the general population would be due to PD.5

Prevalence studies may not accurately reflect the true frequency of PD-D, as demented patients are unlikely to survive as long as nondemented PD cases. Incidence rates are thus more reliable, as they are prospective and relatively free of survival bias. The incidence rate of dementia was reported to be six times higher in PD compared with controls.6 In the Sydney cohort 48% of surviving patients had developed dementia 15 years after the diagnosis,7 the cumulative incidence had risen to 83% 20 years after the diagnosis.8 Similarly, in a UK-based study, the cumulative incidence of dementia was 53% after 14 years.9 Another study conducted in Norway reported that 78% of the patients had dementia at the 8-year follow-up, 26% of cases being demented already at baseline.10 The 12-year cumulative incidence of dementia was also reported for this cohort: by the end of the follow-up period 60% of patients had developed dementia, the cumulative incidence steadily increasing with age and duration of PD.11

Old age at disease onset or at the time of evaluation, long disease duration, and atypical neurological features such as early autonomic failure, symmetrical disease presentation, and unsatisfactory response to dopaminergic treatment ...